1. Field of the Invention
The present invention relates to a material for organic electroluminescence devices and an organic electroluminescence device using the said material, and more particularly to a material for organic electroluminescence (EL) devices providing an electroluminescence device exhibiting a high luminous efficiency, sufficiently reduced driving voltage, high thermal resistance and has long lifetime.
2. Prior Art
There has been an increasing interest in developing novel organic materials that cater to organic light emitting devices (OLEDs). Such devices are commercially attractive because they offer the cost-advantageous fabrication of high density pixeled displays exhibiting brilliant luminance with long life times, high efficiency, low driving voltages and wide color range.
A typical OLED comprises at least one organic emissive layer sandwiched between an anode and a cathode. When a current is applied, the anode injects holes and the cathode injects electrons into the organic layer(s). The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton” which is a localized electron-hole pair having an excited energy state, is formed. Light is emitted when the exciton relaxes through a photoemissive mechanism. To improve the charge transport capabilities and also the luminous efficiency of such devices, additional layers around the emissive layer, such as an electron transport layer and/or a hole transport layer, or an electron blocking and/or hole blocking layer(s) have been incorporated. Doping the host material with another material (guest) has been well demonstrated in literature to enhance the device performance and to tune the chromaticity. Several OLED materials and device configurations are described in U.S. Pat. Nos. 4,769,292, 5,844,363, 5,707,745, 6,596,415 and 6,465,115, which are incorporated herein by reference in their entirety.
The reason for manufacturing an organic EL display with a multi-layered thin film structure includes stabilization of the interfaces between the electrodes and the organic layers. In addition, in organic materials, the mobility of electrons and holes significantly differ, and thus, if appropriate hole transportation and electron transportation layers are used, holes and electrons can be efficiently transferred to the luminescent layer. Also, if the density of the holes and electrons are balanced in the emitting layer, luminous efficiency can be increased. The proper combination of organic layers described above can enhance the device efficiency and lifetime. However, it has been very difficult to find an organic material that satisfies all the requirements for use in practical display applications.
The most important factors to determine luminous efficiency, lifetime in an organic EL device are the appropriate matching of the singlet or triplet energy levels, balanced mobility of electrons and holes, stability of the material during evaporation and thin film morphology, thus leading to the requirement of materials with significantly improved characteristics.